FAQ: Mass Air Flow (MAF) Sensor Issues?

I think a big part of it too is that you, me, and Mr. Riv have our MAF and fuel tables recalibrated and such. The fuel tables and cells within the PCM have changed. I was thinking about that last night. The tune alone could be enough to keep that P0171 away under "normal" conditions, or at least what the PCM considers to be normal after the tune.

Maybe, but in my case this happened before I ever tuned the PCM - 100% stock - that's why I was using a Mini AFC - to fool the PCM into adding more fuel using the stock fueling table.

I think a genuine lean condition by the PCM's definition means that for some reason the O2 sensor reports there is not enough fuel being injected into the cylinders. The 'conditions' for P0171 help to rule out the possibility that this is being caused by faulty MAF, O2, vac leak, etc.

Well, not sure exactly what the logic is, but I can tell you that 90% of the time P0171 sets, there's an air leak somewhere. The other 10% runs towards the MAF, H02S, or a fuel injector/fueling problem. That 10% is no fun to find......

Does that 90% include Rivieras? I know Rivs don't throw some codes as easily as other 3800 powered cars. At the old Regal board, it was common knowledge that installing a 160º t-stat would throw a DTC on the Regal GS, but the Rivieras were programmed differently. Just a thought...

That 90% includes every car on the planet for the most part. Only difference being is that "newer" cars have lean/rich codes for banks 1 and 2, not just "engine lean/rich".

Well P0171 came back with my Dads old sensor installed.

Question: Is this statement form GMForums.com true?

"Oh, I forgot. And one thing you might check for P0171 (if you haven't already heard it 100 times the battery cables. the fuel pump runs off of your alternator. If the charging circuit is intermittently disrupted in any way (or stressed charging a zombie battery) an older fuel pump is the likely candidate to suffer in performance. Clean 'em. Scrape 'em down to clean copper. Peel back a bit of rubber and check under that, too.

And the ground cable from the positive terminal to the ground on the base of your engine is by far the most important one to clean and clean the connection post.It's a bugger to get to on a '96, but ya have to do it."

BMD wrote:

Well P0171 came back with my Dads old sensor installed.

Question: Is this statement from GMForums.com true?

"Oh, I forgot. And one thing you might check for P0171 (if you haven't already heard it 100 times the battery cables. the fuel pump runs off of your alternator. If the charging circuit is intermittently disrupted in any way (or stressed charging a zombie battery) an older fuel pump is the likely candidate to suffer in performance. Clean 'em. Scrape 'em down to clean copper. Peel back a bit of rubber and check under that, too.

And the ground cable from the positive terminal to the ground on the base of your engine is by far the most important one to clean and clean the connection post.It's a bugger to get to on a '96, but ya have to do it."

Short answer: Not exactly; you should be sure to clean all the grounds.

Better answer: Poster's trying to be helpful but I think he just meant to say 'be sure to clean the engine grounds very well' and got kinda carried away (I went and looked at the post on it's site). I read some of his other posts and I think he "talks thru his hat" sometimes, the problem he is currently troubleshooting is a bit difficult, and for whatever reason he's not getting the help/guidance he needs to go after it systematically.

However...

The terminal he is talking about (your riv had one from the factory too) is a grounding stud with a "+" stamped into the end so you can hold it still with a screwdriver while you turn the bolt that holds the ring terminals (on the cables) to it. Thing of it is, that's the ground cable that goes to the starter. And if it is corroded enough it wil lbreak anyway when you try to undo it for cleaning... There are other grounds (at the ICM and on the firewall, and other places) that are as important (and like the ones behind the kick panels, kinda tough to get to) as that ground marked "+."

By the way - in the Riv that "+" ground, it is on the frame rail, passenger side. Mine broke long ago when I was disassembling it to clean it and I replaced it with a stainless steel stud and nuts. I cut a "+" in the top of the new stud with a dremel for the same reason - to have a place for a screwdriver to hold the sucker still if I had to take it apart for a cleaning (I have not yet).

What the poster also did not say, but is a factor here: if your grounds are not clean then there will be what an electrician or engineer calls "a difference in potential" between them. That is a pretty big problem as concerns the electronic devices that may rely on those grounds. The voltage-dependent signals associated with that ground will measure wrong (difference smaller at the bad ground) and the car's computers may act wrong, at least somewhat wrong, as a result depending on which ground is bad. If everything were grounded at one point this would not be a big issue -- but it becomes an issue because single-point ground is not really practical although the signals passed by the car's electronics assume that you have one. The way to eliminate this problem is to be sure the grounds are all clean and tight. And a little NoAlOx or tune-up grease on them, once cleaned, won't hurt -- I noticed by the way they changed the labeling on NoAlOx to read as a general electrical connection compound, not necessarily only for aluminum wiring, but intended to prevent oxidation of electrical connections.

So you might want to check and clean all your grounds.. other posts on this site, and that failing the service manual will help you find them all. Happy hunting...

So, in short, if the grounds are good, a bad alternator that is not keeping the battery performing at its peek, will not cause PCM errors?

BMD wrote:

So, in short, if the grounds are good, a bad alternator that is not keeping the battery performing at its peek, will not cause PCM errors?

Why would you think that?



You can remove/replace an alternator; but you also have to roll up your sleeves and clean/serivce the grounds.

Put another way, the alternator and the grounds are both relatively important parts of the car's electrical system and are among the things that need attention. If the battery's bad it won't charge. If the alternator's bad it won't charge the battery, which will lose charge until it goes bad. If the grounds are bad the electronics are not assured to work right.

The only "out" I know is that as part of high-mileage maintenance (we had a thread on this) we need to include:

- periodic load test of battery to see that it has enough CCAs
- cleaning the grounds every so often (yearly? I dunno)
- periodic load test of the alternator



I am not sure how you check the physical condition of the alternator without removing and disassembly to inspect the slip rings and brushes. The battery you can test with a voltmeter and make some inferences. The grounds you can visually inspect, problem being the chance you'll break some tough to replace stud.

Never seen a bad battery or alternator throw a P0171......................

Abaddon wrote:

Never seen a bad battery or alternator throw a P0171......................

Agreed. My comment answered the imjmediate question but was off the stated topic of this thread (which was resoved some time ago by cleaning the MAF)

I didn't think that a bad alt or bat would throw this code either, but when your scouring the net for answers, you come up with a lot of possibilities and goose chases, thats why I knew that if I asked this question here, it would either be given credibility or be shot down....



thanks guys.

albertj wrote:

Abaddon wrote:

Never seen a bad battery or alternator throw a P0171......................

Agreed. My comment answered the imjmediate question but was off the stated topic of this thread (which was resoved some time ago by cleaning the MAF)

The MAF resolved this code for Aaron, but so far hasn't resolved it for my Dad.

BMD wrote:

I didn't think that a bad alt or bat would throw this code either, but when your scouring the net for answers, you come up with a lot of possibilities and goose chases, thats why I knew that if I asked this question here, it would either be given credibility or be shot down....



thanks guys.

albertj wrote:

Abaddon wrote:

Never seen a bad battery or alternator throw a P0171......................

Agreed. My comment answered the imjmediate question but was off the stated topic of this thread (which was resoved some time ago by cleaning the MAF)

The MAF resolved this code for Aaron, but so far hasn't resolved it for my Dad.

We'll resolve it. Would you like to try making a short list of what's been done to try to resolve the code, since it came up. When I get my manuals back I can review to get ideas. Also when was the last time the codes were cleared/reset - and remind me do you have a scan tool like Car Code or Autotap?

BMD wrote:

Well hes got new.... IGN module, plugs, MAFF, IAC, TPS, oxygen sensor, fuel filter, coolant sensor, we cleaned the throttle body, checked and replaced vacuum lines... the only thing left that I can think of aside from getting the injectors checked is the Fuel Pressure Regulator. In my FSM it says that "If the fuel pressure is too low, poor performance and a DTC P0171, could result" But his car drives really smooth and strong????

Also, since then I swapped out my Dads new O2 sensor with his old one thinking that maybe the new O2 sensor was the culprit but the code returned with the old one installed. My Dad had the bat and alt checked out today and everything is fine . We just now un-plugged the MAF sensor ( second one ) and resest the SES light. When we re-started the car the SES light came on, but this time it was P0102, which is a MAF code, obviously because it is unplugged, but so far, P0171 has not returned. So I am assuming that if P0171 does not return with the MAF unplugged, then we will bring the MAF back for a third.

I do not have an actual scanner. I have a code reader that records freeze frame data.

BMD wrote:

BMD wrote:

Well hes got new.... IGN module, plugs, MAFF, IAC, TPS, oxygen sensor, fuel filter, coolant sensor, we cleaned the throttle body, checked and replaced vacuum lines... the only thing left that I can think of aside from getting the injectors checked is the Fuel Pressure Regulator. In my FSM it says that "If the fuel pressure is too low, poor performance and a DTC P0171, could result" But his car drives really smooth and strong????

Also, since then I swapped out my Dads new O2 sensor with his old one thinking that maybe the new O2 sensor was the culprit but the code returned with the old one installed. My Dad had the bat and alt checked out today and everything is fine . We just now un-plugged the MAF sensor ( second one ) and resest the SES light. When we re-started the car the SES light came on, but this time it was P0102, which is a MAF code, obviously because it is unplugged, but so far, P0171 has not returned. So I am assuming that if P0171 does not return with the MAF unplugged, then we will bring the MAF back for a third.

I do not have an actual scanner. I have a code reader that records freeze frame data.

One of the list members with experience reading MAP values with an autotap or other scanner could be of help here.

albertj wrote:

By the way is it a problem to deal with the MAF gaskets?

There is no paper gasket, just an O ring where the MAF meets the TB.

OK update time. After my Dads no start was cured with a new Ignition Module, we had a P0101 code, which was a MAF code. Car was prone to stalling. We replaced the MAF and even though it wouldn't completely stall it would want to at every warm startup. So we returned the MAF for another one, this one was better but still, there was a stutter at start up. We then did a whole bunch of stuff that is documented in this thread. So we returned the second MAF and bought one from NAPA and this one works flawlessly, so it was the MAF all along and not a fuel pump issue at all! Thanks to everyone for their help.

I will go into detail on the MAF in the https://rivperformance.editboard.com/t6354-code-p0171-lean-condition-solved-dirty-maf-sensor?highlight=P0171 thread.

OK backtrack time. After my Dads no start was cured with a new Ignition Module, we had a P0101 code, which was a MAF code. Car was prone to stalling. We replaced the MAF and even though it wouldn't completely stall it would want to at every warm startup. So we returned the MAF for another one, this one was better but still, there was a stutter at start up. We then cleaned the throttle body, it was coated with black soot, and the throttle plate was sticking slightly. We also put in a new Throttle Position Sensor to go along with the new Idle Air Control Valve, O2 Sensor, PCV valve, Coolant Temperature Sensor, Fuel Filter and Air Filter.

Then we get the P0171 code. At this point we were stumped so we returned the second MAF and bought one from NAPA.

So now the update...

This one works flawlessly. It is a re-man but the parts guy showed two, one that was $100 cheaper and one that was listed under the OEM part number which is the one we got. It seems that the generic one will work on some Series II motors, but for whatever reason my Dads PA required the specific MAF.

It was the MAF all along, so the thread title hold true! Thanks to everyone for their help.

I ran into this same problem couple days ago. After reading alot of the posts, I narrowed my PO171 down to MAF, Vacuum leak, Fuel pressure/filter and possible LIM gackets. I cleaned the MAF with CRC MAF cleaner,after going to the junk yard and buying a NOS replacement. I also unplugged my MAF, which led me to believe I found the problem. When I unpluged it the rough idle and stalling went away. After replacing old MAF with new one, no change. Wasn't that. Next I replaced PVC valve and checked plugs for fouling, plugs were fine, so I assume there is no gasket leak with LIM, although there could be vacuum loss there? I replaced fuel filter which also lead to new sections of gas and return lines, and several hard to locate quik connects for fuel lines. Again, no change, still skippin and stumbling at idle and eventually stalls. I scanned again and found a total of 3 codes next. This after two days of driving under this condition. 2 PO171 System Adaptive fuel too lean., and PO121 Throttle Position Sensor (TPS) range/performance fault. Which I think also explains recent gas smell and flooding at start up? It has also poped thru the TB,and intake a couple times when I started it, and it stalled. My Vacuum/Boost gauge also showed a noticeable loss at around 4-8, normally it ran 12-18. So in my search for vacuum leaks I traced entire vacuum source and replaced all lines,hoses,fittings and caps,sealing each one upon assembly with black RTV, to help cut down on leaks and chances of coming apart under pressure. I started at my vacuum gauge thru the fire wall to a 2 in 1 connecter, which shares the same side as vacuum gauge and then comes out as one vacuum line, to a T, under the nose cone of the super charger. Here it next T's off at the fuel pressure regulator, which I also replaced as suspect when I did fuel filter,lines and fuel injector cleaner,and then travels to the super charger bypass selonoid. It then T's again to meet the super charger bypass valve. and goes to port on top of super charger. This port is, I believe,the main source of vacuum and goes to the brake booster. All new vacuum lines fittings and no change.
While replacing all tyhese lines and fittings I found the source of my problem and it was so simple, so obvious I never even looked! I guess the new parts will ensure some sleep at least! Lol. The EVAP Cannister Purge Solenoid Valve leads to a port on the Throttle Body just below the MAF, this was disconnected from the TB. I used the black RTV to secure the fitting to the line and TB. Problem solved. Runs great again.
My question is, what caused this to come off the TB? Back pressure, something boost related? I noticed one other time There were 2 extra ports on my super charger after the engine swap that had no use on my 98's set up. It was from a 97. I used vacuum caps to cap both the unused ports and it would eventually cause them to pop off and run eratic. I tyhen connected the 2 ports to each opther using one line and two elbow fittings. No issues. Is this ok to run this way, 2 ports in a loop ? And again what would cause the caps to pop off like the EVAP/TB line did? Any help would as always be greatly appreciated.

I took pictures as i went thru repair, and some today to show source of my specific PO171 code related problem. While doing so I broke the fitting! That RTV really holds stuff in place! I will add pictures at another time. Fixing this again and then custom steering wheel cover tonite!

loop between 2 capped fittings: short answer is I Don't Know and I kinda doubt it. .

Long answer is that loop does not equal separate caps in some cases. If cap 1 is holding against a vacuum of 2 psi and cap 2 is holding against a vac of 4 then you could have (at least in theory) a 2 psi bleed into the 2 psi port, depending on the system. That is, what is pulling that 2 psi, how constant is that pull, and if you pull against it does it bleed from somewheres else?

In some cases it does not matter, there's no bleeding anywhere and you end up with 4 psi vac all around. (in the real world this is almost never the case). In other cases if you have a sensor that is supposed to read that 2 psi vacuum then the sensor will get the wrong reading.

It's a relatively straightforward physics problem. But w/o a vac diagram to show the entire vac circuits, knowledge of the devices in the system (what's pulling the vac?) and state diagrams showing values in different states of operation I just don't know.

For me the simple answer is given GM's engineering you can bet that separate caps were the least expensive way to bring a workmanlike solution to the problem and it may or may not be the case that looping them is OK. I mean, it could be OK and then something else leaks and you now have a vac-related problem that the documentation (the service manual) does not anticipate.

Albertj

Albert, thank you for the input. I will dig deeper as time permits. I still cant figure out what caused the caps to pop off? Was it driving conditions,elevation, some sort of surge or reverse of vacuum somehow causing back pressure. Idk just has me baffled, and everything has a reason! Lol. Just curious if it is a sign of something prepairing to fail?

curious riv wrote:

1-goes to the MAP sensor
2- goes to the LIM
3- goes to the fuel pressure reg(i think)
4- goes to the thingy right beside the bypass valve

Sorry about resurrecting this old thread... I threw a PO102 today at the lake, after experiencing an idle rpm surge. First time the engine died, restarted fine, then RPM surged again like 3 times in 5 seconds, but kept running and smoothed out, but the SES came on. Was able to drive home 1.5 hrs with little issue, except one instance where it partially cut out at highway speeds a few times over a few minute stretch. But mostly ran ok getting home. Got home and found only the PO102 code. Pulled the MAF sensor and cleaned it, though it looked clean. Checked vacuum connections, didn't find anything obvious. Engine did idle ok when I got it home in the driveway. After cleaning the MAF sensor and checking connections, it started and idled without issue, so MAYBE problem solved...

However, I do have a spare new vacuum line as pictured here. Would like to swap it in, but for the life of me I can't locate where it hooks up around the backside of the engine! I can trace the line around the backside as it runs along the top of the rear valve cover, but can't locate the end connection. It seems there's just too much stuff in the way to find/locate it, visually or manually. Are there specific items that need to be removed in order to get at the rear vacuum connection?

It's the MAP sensor on the backside of the engine, the metal nipple of the sensor points down and is not visible unless you have an inspection mirror (or remove the sensor ha ha ha)

If you just go ahead and put the line in, the rubber fitting will be **near** the nipple on the MAP.

you'll want that mirror...

Albertj

Geez, could they have made it any harder? I knew it belonged on the MAP sensor, just couldn't see any good way to get at it. Don't know how I'm going to get a mirror in there, can't even get a hand back there to find the nipple. Car is running fine so far today, maybe cleaning the MAF sensor and rechecking the vac connections solved the issue.... for now... MAF sensor shouldn't have needed cleaning though. Thought I had a spare on an extra TB I have, but there's no sensor on it.

When I replaced my vac line I did not use a mirror to find the MAF nipple, just did it by feel and by remembering where I pulled the line from in the first place. Problem is there are a couple bolt ends sown there that feel like the MAF nipple. You need to get the $(#*@)$ vac-line on there or eventually you'll set a code and run like gassy poopy. The line threads in fine, you'll find an opening right about where it should be once you put the vac line on the rest of the nipples.

Well I'm not pulling off the old line just yet. Not until I can physically locate the MAP nipple. My fingers just don't seem to be able to follow the line far enough to reach the end. May not be necessary anyway. And my engine bay is not exactly grease and grime free, so won't be pulling any lines off any nipples without doing some cleaning first. Don't really want to have to mess with that in our current cold weather.

I spoke too soon earlier today when I stated that it seemed I had solved the problem with a MAF cleaning and rechecking vac lines. Set the code again at noon. Tonight I stopped by NAPA and picked up an AC Delco reman MAF sensor and swapped. It SEEMED to make a noticeable difference in the general idle quality, and no codes or engine surging after 15min driving around town. Will see if that holds true tomorrow.